Remove your SIM card from the SIM card tray and then put the SIM card back. Make sure that the SIM tray closes completely and isn’t loose. If you use a SIM tray from a different model iPhone or from another phone manufacturer, it might not fit properly.

Try using another SIM card. If you don't have one, go to your carrier's retail store and ask them to test with another SIM card. You might need to replace your SIM card.